Since it became clear that the wavelength chirp present with direct modulation of a laser source would severely limit the possible modulation band-widths available for long-wavelength (around 1.5 .mu.m) optical communication, there has been great interest in developing high-speed electroabsorption and electroreflectance modulators. This has led to the design of a number of modulator and integrated laser-plus-modulator structures based on the quantum confined Stark effect (QCSE). The quantum well layers of such quantum well semiconductor structures are typically single composition layers possessing no substructure, and hence providing wells that are substantially flat-bottomed and square-sided. There has, however, also been some investigation of the properties of quantum well layers that do possess a substructure, such a substructure being provided to produce a more complicated well profile. Thus EP 0 416 879 proposes a QCSE modulator whose quantum well layers have a substructure comprising two substructure layers of different material designed to provide the well with a built-in step that produces a dipole at zero applied electric field, and in consequence is designed to produce applied field induced band edge shifts without any significant change in absorption coefficient. A somewhat similar structure is described by T. Tutken et al in a paper entitled, "Large observed exciton shifts with electric field in InGaAs/InGaAsP stepped quantum wells", App. Phys. Lett. 63 (8), Aug. 22, 1993, pp 1086-1091, whose authors are concerned to maximise the QCSE shift for a given value of applied field. The effects upon various parameters, including Stark shift, exciton binding energy, and overlap of electron and hole wavefunctions, produced by changing the shape of quantum wells, have been made the subject of study in a paper by W. Chen & T. G. Anderson entitled "Quantum-confined Stark shift for differently shaped quantum wells", Semicond. Sci. Technical 7 (1992) pp 823-836. This paper treats not only substructures providing 2-step and 3-step wells, but also wells with profiles that are partly or wholly continuously graded. A quantum well substructure can also be employed to provide a QCSE device that exhibits a blue-shift of absorption edge instead of the red-shift exhibited by QCSE devices of more conventional type. Such a blue-shift device is described by P. N. Stavrinou et al in a paper entitled, "Use of a three-layer quantum-well structure to achieve absorption edge blueshift", App. Phys. Lett. 64 (10), Mar. 7, 1994, pp 1251-1253. Blue-shift resulting from a non-uniform composition quantum well is also described by W. Zhou et. al. in a paper entitled, "Simultaneous blue- and red-shift of light-hole and heavy-hole band in a novel variable-strain-quantum well heterostructure", App. Phys. Lett. 66 (5) Jan. 30, 1995 pp 607-609. This describes a strained quantum well with a graded composition providing a value of strain that is graded in magnitude from one side of the well to the other. The paper explains that this grading makes the device polarisation controllable, providing it with the property that, at a unique value of applied bias, there is a cross-over between the red- and blue- Stark shifts for the heavy- and light-hole transitions.
In an integrated laser-plus-modulator the state of polarisation of the laser light that is incident upon the modulator is fully determined, and this may also be conveniently arranged to be the situation in the case of laser-plus-modulator configurations that are not integrated. Under these circumstances matters can usually be arranged so that any polarisation sensitivity exhibited by the modulator is of no practical consequence. On the other hand, there are other applications of modulator structures, such as pulse train shaping, where the polarisation of the light beam is unknown. There have therefore been attempts to design polarisation-insensitive quantum confined Stark effect (QCSE) modulators for these `between fiber` applications. In the long wavelength range (around 1.5 .mu.m), studies have been made of InP-based devices containing InGaAs layers with a modest amount of tensile strain (less than 1% lattice mis-match). With the appropriate strain for the well thickness, the E1-HH1 transition (which is responsible for most of the TE absorption) and E1-LH1 transition (which is responsible for TM absorption) can be made degenerate at zero bias. This has been shown in the paper by T. Aizawa, K. G. Ravikumar, S. Suzaki, T. Watanabe, and R. Yamauchi, "Polarisation-independent quantum confined Stark effect in an InGaAs/InP tensile-strained quantum well", IEEE Journal of Quantum Electronics, 1994, 30, pp. 585-593, to correspond to equal TE and TM absorption of the incident light beam at zero bias, and hence to polarisation-insensitivity. However, as a field is applied across these structures the E1-LH1 and E1-HH1 transitions show different Stark shifts and do not remain matched because the shifts depend on the effective masses. As a result, existing modulator structures based on the QCSE are only truly polarisation insensitive at one field strength.